Non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries are widely used as batteries for use in personal computers, cellular phones, portable information terminals and so on because of their high energy density.
Non-aqueous electrolyte secondary batteries as represented by these lithium ion secondary batteries have high energy density, and when an internal short-circuit or an external short-circuit occurs due to damage of a battery or damage of the device employing the battery, a high current flows, and severe heat generation occurs. For this reason, it is demanded to prevent heat generation over a certain degree and to ensure high safety in a non-aqueous electrolyte secondary battery.
As means for ensuring such safety, a method. of imparting a shutdown function that blocks passage of ions between the positive and the negative electrodes by a separator in case of abnormal heat generation to prevent further heat generation is generally employed. As a method for imparting a shutdown function to a separator, a method of using a porous film formed of a material that melts in case of abnormal heat. generation as a separator can be recited. That is, in the battery using this separator, the porous film melts and loses pores in case of abnormal heat generation, blocks passage of ions, and thus can prevent further heat generation.
As a separator having such a shutdown function, for example, a porous film of polyolefin is used. The separator formed of a polyolefin porous film melts at about 80 to 180° C. to lose pores in case of abnormal heat generation of the battery, and thus it blocks passage of ions (shutdown) and inhibits further heat generation.
By laminating a layer containing a polymer other than polyolefin, functions such as shape retentive property under heating at a temperature higher than the melting point of polyolefin, and adhesiveness with electrodes can be imparted to the polyolefin porous film. As a laminated porous film formed by laminating a layer containing a polymer other than polyolefin on the polyolefin porous film, for example, a laminated porous film that is produced by laminating a layer containing cellulose ether and fine particles on a polyolefin porous film, to which shape retentive property under heating at a temperature higher than the melting point of polyolefin is imparted (Patent Document 1), and a laminated porous film that is produced by laminating a layer containing polyvinylidene fluoride on a polyolefin porous film, to which adhesiveness with electrodes is imparted, having excellent uniformity of the electrode-to-electrode distance (Patent Document 2) are proposed.